1. Field of the Invention
The present invention relates to a particle measuring device for measuring dust in semiconductor clean room applications. More particularly, the particle measuring device uses a funnel-shaped collector to concentrate the air sample such that a larger air sample is measured and the sensitivity of the device is improved.
2. Background of the Related Art
A clean room is an interior area with provisions to control and reduce airborne particles to the very low levels required for the manufacture of semiconductors and other such processes. Airborne and suspended particles are controlled so that they cannot reach the object being worked on within the clean room. Also, optimum process conditions for the work object can be obtained by controlling air temperature, humidity and illumination. Measures for preventing noise and vibration are also taken where this will adversely affect the manufacturing process within the clean room.
This is especially true in a semiconductor production line, where a series of processes, such as basic pattern design, layer formation, reticle manufacture, wafer manufacture, inspection, assembly, packaging, final testing, and quality inspections, etc. are executed. During the wafer manufacture process in particular, since exposing, developing, etching and diffusion processes are repeatedly executed, control of dust pollution, temperature and humidity are all very important. These factors are significant to product yield and to the various aspects of improving the precision and reliability of the final products.
Accordingly, pollution management of clean rooms in semiconductor manufacturing processes is very important. In order to properly execute pollution management, it is important to remove dust from the air flowing into the clean room and to prevent pollution sources from being introduced to the clean room. In addition, it is also important to measure the air cleanliness of the clean room, and to manage the clean room by utilizing filtering systems and air conditioning systems on the basis of the measured data.
For pollution management of a clean room, it is necessary to measure the degree of pollution at a position within the actual clean room, using samples taken at various intervals. For this, a particle measuring device is generally used. A suspended particle measuring device is used to measure the cleanliness of the actual clean room from a sample taken from the clean room. Several kinds of suspended clean room from a sample taken from the clean room. Several kinds of suspended particle measuring devices have been variously developed for this purpose and are commercially available.
One such device being used is an optical particle counter for measuring the pollution by measuring the scattering level of a laser beam that occurs from the presence of dust particles in its path. A typical configuration of this device includes a laser light source which generates a laser light beam and a cut-off wall which is installed opposite to the laser light source that cuts off the laser light. In addition, a light sensor is placed perpendicular to the laser beam to measure the light scattered from the laser beam by the airborne dust particles.
Such a conventional particle measuring device is depicted schematically in FIG. 1. A pump 2, having a flow meter 3 attached to one side, is connected to the above-described particle counter 1. The pump 2 circulates fresh air to be measured by the particle counter 1. An entry pipe 5 is also connected to the particle counter 1 to allow the air sample to be drawn therethrough. The air sample is gathered through a directional suction pipe 7, which is connected to the entry pipe 5 by the flexible connection tube 6 at one end. At the other end of the directional suction pipe 7, a wide suction entrance 8 is formed. Therefore, the air sample to be measured is drawn into the entry pipe 5 by the operation of the suction pump 2. Essentially, the particle counter 1 measures the air sample, and the air sample being measured is discharged to the clean-room. In this way, the particle counter 1 measures the quantity of dust particles suspended in the clean room where the air sample was taken.
The particle counter 1, the pump 2 and the flow meter 3 are all known and commercially available, so they will be easily understood to those skilled in this field of technology.
A cleaning filter is placed on the discharge portion of the pump 2, so that additional pollution due to the operation of the particle counter 1 itself does not occur. The connection tube 6 may be a hollow flexible tube made of TEFLON or similar material suitable to this application.
A disadvantage of the conventional particle counter 1 is that it is limited by the size of the air sample to be measured. Since clean rooms are generally getting larger, the relative size of the air sample in comparison with the air contained within the clean room is very small. As a result, the reliability of the measurement decreases.
For example, clean rooms for precision electronics industries, such as a semiconductor production lines, have became larger with heights on the order of 3.5 meters, widths of 100 meters and lengths of 100 meters to facilitate improvements in productivity. Despite circulating the fresh air more than four hundred times per hour to maintain the required high cleanness, the amount of the air sample obtained during one measurement of the particle counter 1 is only 28.3 liters. Note that the measurement sample is 1/1.27.times.10.sup.-7 of the total room volume. It is not very reasonable to manage the whole clean room on the basis of such a small measurement.
On the other hand, although the deviation in measuring data may be reduced by increasing the number of measurements, the amount of the measurement sample is still small in comparison to the total air volume of the whole clean room. Furthermore, the measurement process itself takes considerable time, rendering real time measurements difficult.
Some have tried to increase the sample size so as to decrease the sampling time. But as shown in FIG. 2, there is an inverse relationship between the amount of the measured air sample and the minimum measurable particle diameter. For example, with an ordinary particle counter 1 using a He--Ne laser (helium-neon laser), the minimum measurable particle diameter is only 0.09 .mu.m in the measured sample air of 28.3 liter. Therefore, this second approach is not of much practical use.